1. Field of Invention
The present invention generally relates to structures constructed from blow-molded plastic and, in particular, to high-strength, lightweight blow-molded plastic structures.
2. Description of Related Art
It is known to construct various objects and items, such as tables, chairs, partitions, walls, and sports equipment, from plastic. It is also known to construct some of these and other objects from blow-molded plastic. As known to those skilled in the art, blow-molded structures include an outer wall that encloses a hollow interior space. Because blow-molded structures include a hollow interior space, many blow-molded structures are not high-strength. In fact, many known blow-molded structures are relatively low-strength and are unable to support a relatively large amount of weight or force.
In order to increase the strength of conventional blow-molded structures, it is known to add strengthening ribs. For example, known tables with table tops constructed from blow-molded plastic may include strengthening ribs attached to the underside of the table top in an attempt to increase the strength of the table top. The strengthening ribs are often constructed from metal because metal ribs are high strength. Disadvantageously, the metal ribs must be attached to the table top by mechanical fasteners such as bolts or screws. The bolts or screws require holes in the table top, which may create stress concentrations and/or potential failure points in the table top. Alternatively, the metal ribs may be attached to the table top by adhesives, but the adhesives may also decrease the structural integrity of the table top and make the table top very difficult to repair or replace. In addition, the metal ribs increase the number of components required to construct the table and increase the time required to manufacture the table. Thus, while it is known to use metal ribs to increase the strength of a blow-molded table top, metal ribs undesirably increase the time and costs to manufacture the table. Further, the attachment of the metal ribs to the table top may decrease the strength and/or structural integrity of the table top, and increase the potential causes of failure of the table.
Conventional table tops constructed from blow-molded plastic may also include strengthening ribs or beams that are integrally formed in the table top. For example, strengthening ribs may be formed in the underside of the center of the table top to prevent sagging. These strengthening ribs are generally large, elongated portions that extend along the length or width of the table top. The strengthening ribs may increase the overall strength or structural integrity of the table top, but the ribs require thicker outer walls so that the large, elongated ribs are correctly formed in the table top. The large strengthening ribs also require thicker outer walls so that the ribs do not undesirably sag or deform during the manufacturing process. Disadvantageously, the thicker outer walls of the ribs require additional plastic materials be used to create the table tops, which increases the costs and weight of the table tops. In addition, the thicker outer walls retain more heat during the manufacturing process. Thus, a longer cooling time is required during the manufacturing process in order to allow the thicker outer walls to cool. This undesirably increases the time of the manufacturing process because the blow-molded table tops cannot be removed from the mold until the tops are sufficiently cooled.
Known table tops constructed from blow-molded plastic may include several large strengthening ribs to strengthen various portions of the table top. For example, strengthening ribs may be placed near the opposing ends of the table top in order to increase the strength of the ends of the table top. Additionally, as discussed above, strengthening ribs may be placed near the center of the table top to help prevent the table top from undesirably bending and to allow the table top to support additional weight. These strengthening ribs often protrude downwardly from the underside of the table top and the ribs often have large profiles to create relatively strong supporting structures. Disadvantageously, the large, protruding ribs may undesirably decrease the amount of room underneath the table top and limit the potential design considerations for the table top.
Conventional table tops constructed from blow-molded plastic and including strengthening ribs may still not have sufficient strength and may allow a portion of the table top to sag. In order to increase the strength of these conventional table tops, it is known to increase the number of ribs. These additional ribs require a larger area on the underside of the table top, which limits the area that other features may be formed in the table top. The plurality of strengthening ribs may also interfere with other desired features or components of the table, such as interfering with the folding of the table legs into a collapsed position. The ribs may also further increase the cooling time and manufacturing time of the table top.
Additionally, while the large strengthening ribs may prevent large portions of the table top from sagging, the ribs may allow smaller, localized portions of the table top to sag. In particular, because the distance between the table top and the bottom portion of the rib is greater than the distance between the upper and lower surfaces of the table top, this may allow localized portions of the table top to sag. Additionally, because the strengthening ribs are large and have an elongated length, they support the localized portion of the table top differently than the other portions of the table top. Thus, the upper surface of the table top may be uneven because different portions of the table top are supported differently.